In Situ Strain Measurements in Libya

Abstract

For the first time, in situ rock strain measurements have been carried out in Libya. Except in Liberia there are no previous in situ stress results of the African continent north of the equator. In this paper I report the results of stress determinations at 26 sites that extend from the Tunisian-Libyan border to Derna/Cyrenaica in the north and to Ghadames and the Jufrah oasis in the south. The present rock strain has been determined by overcoring of strain gauges that were bonded to the rock surfaces. The range of rock ages selected for measurements was wide (Middle Triassic to Quaternary), but their lithologic character was similar (solid homogeneous micrites). The orientations of maximum and minimum horizontal stress revealed a consistency across large areas. Most stress values vary from 0–50 bars; many range between 50 and 150 bars, and at one site, the stress was tensile in all horizontal directions. At least three crustal domains in northern and central Libya can be defined by means of different in situ stress fields. The western Libyan stress domain extends from Tunisia into Libya and is terminated along a line that runs from Sabratah to Azizia, Bu Ngem and the Jabal Waddan east of the Hon graben. This western Libyan stress field has a NW — SE-oriented maximum compressive stress that is horizontal. To the east, the central Libyan stress domain has a NE — SW-directed horizontal maximum compressive stress component that incorporates the entire Sirte basin and major parts of Cyrenaica. A third in situ stress domain occurs along the Cyrenaican coastal area from Al Beda to the east. There, the stress field has a maximum compressive component of NW — SE-direction. It is suggested that Tripolitania and the eastern Cyrenaica and their lithospheric northern extensions are indenting the European plate in a northwestern direction corresponding to the drift direction of the African plate for the last 9 m. y. The Sirte basin is located between those framing indenters and was subjected to extensional tectonics during the late Mesozoic and most of the Tertiary, but may have been under NE — SW-directed horizontal compression since the late Neogene.